Mohammad Ebrahimi

Dynamic Model and Control of DFIG Wind Energy Systems Based on Power Transfer Matrix

This paper presents a power transfer matrix model and multivariable control method for a doubly-fed induction generator (DFIG) wind energy system. The power transfer matrix model uses instantaneous real/reactive power components as the system state variables. It is shown that using the power transfer matrix model improves the robustness of controllers as the power waveforms are independent of a dq frame of reference. The sequential loop closing technique is used to design the controllers based on the linearized model of the wind energy system. The designed controller includes six compensators for capturing the maximum wind power and supplying the required reactive power to the DFIG. A power/current limiting scheme is also presented to protect power converters during a fault. The validity and performance of the proposed modeling and control approaches are investigated using a study system consisting of a grid-connected DFIG wind energy conversion system. This investigation uses the time-domain simulation of the study system to: 1) validate the presented model and its assumptions, 2) show the tracking and disturbance rejection capabilities of the designed control system, and 3) test the robustness of the designed controller to the uncertainties of the model parameters.

A New Approach for Induction Motor Broken Bar Diagnosis by Using Vibration Spectrum

Different methods for detecting broken bars in induction motors can be found in literature. Many of these methods are based on evaluating special frequency magnitudes in machine signals spectrums. Current, power, flux, etc are among these signals. Frequencies related to broken rotor fault are dependent on slip; therefore, correct diagnosis of fault depends on accurate determination of motor velocity and slip. The traditional methods typically require several sensors that should be pre-installed in some cases. A practical diagnosis method should be easily performed in site and does not take too much time. This paper presents a diagnosing method based on only a vibration sensor. Motor velocity oscillation due to broken rotor causes frequency components at twice slip frequency difference around speed frequency in vibration spectrum. Speed frequency and its harmonics as well as twice supply frequency, can easily and accurately be found in vibration spectrum, therefore the motor slip can be computed. Now components related to rotor fault can be found. According to this method, an apparatus consisting necessary hardware and software has been designed. Experimental tests have confirmed the efficiency of the method

Fault Diagnosis of Bearings in Rotating Machinery Based on Vibration Power Signal Autocorrelation

Since fault in a great number of bearings commences from a single point defect, research on this category of faults has shared a great deal in predictive diagnosis literature. Single point defects will cause certain characteristic fault frequencies to appear in machine vibration spectrum. In traditional methods, data extracted from frequency spectrum has been used to identify damaged bearing part. Because of impulsive nature of fault strikes, and complex modulations present in vibration signal, a simple spectrum analysis may result in erroneous conclusions. When a shaft rotates at constant speed, strikes due to a single point defect repeat at constant intervals. Each strike shows a high energy distribution around it. This paper considers the time intervals between successive impulses in auto-correlated vibration power signals. The most frequent interval between successive impulses determines the period of defective part. This period is related to fault frequency and therefore shows the defective part. A comparison of results extracted from the traditional and the proposed methods shows the efficiency improvement of the second method in respect of the first one